1. FIELD OF THE INVENTION
This invention is broadly concerned with an apparatus for controlling a heating temperature and intended particularly to be used to heat resin under various conditions in an injection mold machine or an extruder.
2. DESCRIPTION OF THE RELATED ART
Conventionally, a heating device is employed in an injection mold machine, an extruder, or the like for melting a resin material. An electric heater is generally used as a typical heating means and is accurately controlled depending upon the type of resin and a molding condition.
One example is schematically illustrated in FIG. 4. An injection mold machine 1 consists of a barrel 2, a screw 3 disposed inside the barrel 2 such that its axle axis is aligned with that of the barrel 2, and an electric band heater 4 on the peripheral surface of the barrel 2. In the operation of the machine 1, a pellet of resin material is put into a hopper 5 and transmitted through the barrel 2 while being heated and melted by the heater 4, and while being compressed and kneaded by the screw 3. The molten materials thus extruded from a nozzle 6 of the barrel 2 and subsequently injected into a die 7 to be molded.
Such an injection mold machine 1 is controlled by a control device 10. The control device 10 functions to synthetically control the rotation of the screw 3 and the temperature of the heater 4. The device 10 includes an operation setting portion 11 and a temperature setting portion 12, for presetting a certain state of the injection mold machine 1, and a control panel 13 for manual control by an operator.
The temperature setting portion is connected with PID controllers 14 to regulate the electric power for heaters 4 and with sensors 15 to detect a real temperature for the barrel 2.
Temperature control by the control device 10 can be schematically described by the block diagram shown in FIG. 5. In the drawing, r(t) is a desired set temperature of the temperature setting portion 12, and control input u(t) is a heating control from the PID controller 14 to the heater 4. Controlled variable y(t) is the real temperature of the barrel 2 based on the heating control and varies with the barrel 2 in accordance with the object to be heated and sorts of resin materials fed into the barrel 2. The real temperature of the barrel 2 detected by the sensor 15 is negatively fed back to an input of the PID controller 14, thereby enabling feedback control to be carried out so as to decrease a deflection e(t) of a set temperature and real temperature. Accordingly, the barrel 2 is set at a predetermined set temperature by the heater 4.
The conventional apparatus shown in FIG. 4 has three lines each consisting of the heater 4, the sensor 15, and the controller 14 toward three portions of the barrel 2; a base portion, a middle portion, and a forward portion. These lines are respectively controlled at a certain set temperature by the temperature setting portion 12 to conduct a zone control of the barrel 2.
It is known that if the temperature of a resin material is low, its moldability is likely to decline, and if it is high, decomposition may result. Hence, temperature control of the barrel 2 is necessary for executing a desirable molding with reduced temperature fluctuation.
However, the conventional injection mold machine 1 can not prevent a delay of the temperature control in response to some alterations of conditions when molding. Since feedback control such as the PID control is employed for the control device 10, the heating control for the heater 4 is always done after detecting some change of the temperature of the barrel. The fluctuation of the temperature becomes rather severe due to the delay of the temperature control, so that the resin in the barrel 2 is not melted enough or decomposed due to excess heating.
Moreover, when changing the set temperature of the injection mold machine 1, or when changing the operating condition of the machine 1, a slight deviation of the real temperature of the barrel 2 is caused.
This is because while heating up the temperature of the barrel 2 with no resin material therein, the temperature is easily changed with a small quantity of heat. But, in an operation in which the resin fed into the barrel 2 is melted and not extruded, it is necessary to change the temperature to apply a greater quantity of heat to the barrel 2. In a molding cycle operation, in which the resin is continuously melted and extruded from the barrel 2, a large amount of heat is necessary to process the resin material successively fed into the barrel 2. Otherwise, the necessary quantity of heat can be changed depending upon an operation cycle of the machine 1.
Hence, when shifting the operation from the preparation step to the molding cycle operation, the necessary quantity of heat increases and the temperature of the barrel 2 decreases. During the molding cycle operation which needs a relatively large quantity of heat, the fluctuation of the temperature tends to be greater because of the delay of heating control, and faulty moldings are produced.
In FIG. 5, such a situation is shown as a disturbance d(t). It can be seen that the disturbance d(t), because of alterations of setting required when starting and stopping molding, causes temperature fluctuation.
Some instances conducted by the conventional apparatus are shown in FIGS. 6 and 7. FIG. 6 is a graphical representation of temperature fluctuation at the time when the molding cycle operation begins at time T1 described by a dotted-line. Incidentally, the set temperature is, for example, 180 degrees. Before the beginning of the molding cycle, the control input (A) is enough to keep the well-balanced condition whereas, in the molding cycle, the control input (B) is required. It can be understood that the difference (A-B) is likely to equal to a stepping disturbance begun at the very moment described by a dotted-line in the drawing. The feedback control takes a long time to cure disturbance and the temperature fluctuation may result.
Referring now to FIG. 7, this is a graphical representation of temperature fluctuation when the molding cycle operation is suspended at the time T2 described by the dotted-line. The setting temperature is kept constant at for example 180 degrees. It is the other way around in this case, that is the value B can balance in the molding procedure whereas the value A can balance in the suspended state.
In order to settle these disadvantages, the Japanese Patent Laid-open No. 3-164224 has proposed another apparatus for controlling a heating temperature. The proposed apparatus is provided with a condition compensating means for adding a compensating input to a control input of heating means such as a heater.
However, it was observed that the compensating data should be preliminary memorized in a memory portion after some examples and various analysis which require a great deal of time for every operation condition.
The present invention aims to provide an apparatus for controlling a heating temperature which does not require to memorize compensating data preliminary and provides a prompt correspondence to temperature fluctuation upon operation states, thereby controlling the temperature accurately and stably.